1. Field of the Invention
This invention generally relates to thermal or heat-sensitive recording for recording an image, such as a character, on a recording medium using a thermal print head, and in particular, to a structure for storing a pattern of a particular image to be printed for use in driving a thermal print head including a single array of heat-producing elements.
2. Description of the Prior Art
Thermal printers are well known in the art. Typically, a thermal printer includes a thermal print head provided with an array of heat-producing elements which are activated selectively in accordance with an image signal thereby recording an image on a recording medium while the thermal print head and the recording medium are moved relative to each other. For low speed recording, serial print-type thermal printers having a movable print head, which moves along a platen roller in a reciprocating manner, have been mostly used; on the other hand, for high speed recording, line print-type thermal printers having a stationary print head with a single array of heat-producing elements extending across the full width of a recording medium, which is moved relative to the print head in the direction normal to the array, have been mostly used.
A thermal print head for use in a thermal printer includes a plurality of heat-producing elements, typically electrical resistors, which are arranged in an array and which are activated selectively to produce heat which is then applied to a recording paper directly to form a "burn" or darkened spot, if the recording paper is heat-sensitive in nature, or through heat-sensitive tape interposed between the print head and the recording paper, if the recording paper is plain paper. In this manner, during recording, each of the heat-producing elements is activated and deactivated repetitively. Such a thermal print head has a particular thermal time constant which is determined by such parameters as the material used and the structure employed. Thus, the timing of activation and deactivation must be determined in consideration of this thermal time constant.
Stated more in detail in this respect with reference to FIG. 1, it is assumed that each of the heat-producing elements provided in the form of an array on a thermal print head is activated for time period t.sub.1 and then deactivated for time period t.sub.2, and, thus, time period t.sub.1 corresponds to heating cycle and time period t.sub.2 corresponds to cooling cycle. Under the condition, if cooling time period t.sub.2 is set too short, the next heating cycle starts before the heat-producing element has cooled sufficiently, so that the peak temperature at the end of the next heating cycle will become higher than that of the preceding cycle as indicated in FIG. 1. Since the density of a recorded image increases as the level of temperature produced by the heat-producing element increases, the density of a recorded image gradually increases as recording proceeds.
In order to cope with such a situation, there has been proposed to shorten time period t.sub.3 of the next following heating cycle as compared with time period t.sub.1 ' of the preceding heating cycle as shown in FIG. 2. With such a scheme, the peak temperature may be maintained at constant even if the heat-producing element is activated successively. Several methods have been proposed to control the activation time period by comparing the image data in two successive lines. For example, according to a technique disclosed in Japanese Laid-open Patent Pub. No. 58-94485, in order to correct the thermal hysteresis from the preceding line, a longer heating time period is provided if the heat-producing element is to be activated for the first time and a shorter heating time period is provided if the heat-producing element is to be activated for the second or later time in succession, thereby allowing to obtain a printed image of uniform density and to prevent the heat-producing element from being overheated. In this case, the activation time period is determined line by line by comparing the image data in two successive lines. Such a technique, however, is rather limited in speed.
FIG. 3 shows a character pattern of number "2" stored in a character generator. Such a character pattern is typically stored in a read only memory in the form of 0s and 1s. That is, in the illustrated example, the character pattern is defined in the form of a matrix having 7 rows and 5 columns with the shaded areas indicating the corresponding heat-producing elements to be activated thereby forming "burn" points on recording paper. In FIG. 3, "M" indicates the main scanning direction which coincides with the longitudinal direction of single array of heat-producing elements and "A" indicates the auxiliary scanning direction in which there exists relative movement between the array and the recording paper, as well known for one skilled in the art. When this character pattern is used with the above-described prior art technique, it is so controlled that the longer heating time period is provided for the data at the intersection between column 5' and row 2 but the shorter time period is provided for the data at the intersection between column 1' and row 2 because the heat-producing element is activated for the second time in succession for the latter data. However, as described above, this is disadvantageous because comparison of data between the two successive lines must be carried out line by line.